They will know fine what they are being brought in for initially.
Any young defender will know Pique is a starter and Umtiti has done well.
Where have I said 'cant sign players as back ups'? In the part you quoted I said it is not impossible but many wont come for that scenario.
Many won't come if you do a lousy job trying to convince them. Like proposing them to play in the 3rd league. That's arrogant and a piss poor job at trying to land a top talent. Here is where the difference lies. Many top talents would come to Barca to fight for a place in the team. We just send them the wrong singals, while, on the other hand, Madrid makes them feel valued. This is why they can field 2 teams that would make CL places in Spain, and we can't.
You don't tell them, players and their agents are usually smart enough to know situation. You don't have to tell young forwards that it would be almost impossible to get much playing time behind MSN. You don't have to tell young CBs that it would be very hard for them to break Pique-Umtiti partnership as our starting CB. You didn't need to tell midfielders back in Xaviesta days that they'll have absolutely no chance to start ahead of them.
Kovacic has no chance to start in Madrid's midfield. But they managed to get him. It's the power of persuasion that a club has. We are fucking Barcelona. If we can't sign a player we want then we are just bad at it. OK, I can understand a top player not wanting to come when we had Xavi and Iniesta, but a young player could have done that. There are pluses as well. You can learn from the best, you can play with all time great players, and develop your own skill by doing that. There are numerous advantages in being at a club like Barca.
Our defense is nothing extraordinary btw. A confident player would come to try to impose himself in the team, if he feels that he can do that at the club. But if you start by telling him to play football against Sabadell and Eldense (I didn't even know these teams until I googled them 1 min. ago), alarm bells are ringing and he will find another team that will give him more chances.
We just used Mathieu in a CL quarterfinal with Juve. There is room allright in our defence. We are just bad at going after the right players. And we when we do, we are bad at trying to convince them to join.